mixin sytax changed

@ProLoser points me they where using dashed mixin syntax and bootstrap use this. So I changed.

Note that when you use a "super-mixin" (as .flex-container) it doesn't fill the css rules with all the fuctions by default. This is why 
".flex-align-content(@align_content) when (iskeyword(@align_content)) "
so the supermixin by default has @align_content:0 and doesn't adds any unwanted stuff.